This invention relates to bias circuits, and in particular to a bias circuit for a microwave FET.
Field Effect Transistors (FETs) have in recent years been replacing IMPATT diodes and other types of semiconductor devices in microwave amplifiers and oscillators. In a typical application, the FET which is driven by the microwave signal is biased by means of a differential amplifier which keeps the source-to-drain voltage and source current essentially constant. This is accomplished by applying the output of the differential amplifier to the gate electrode, while coupling one input of the amplifier to a reference level potential and the other input to a means, such as a resistor, for monitoring the source current. Changes in the source current unbalance the amplifier resulting in a suitable change in the potential applied to the gate to keep the source-to-drain voltage and source current essentially constant.
When a microwave signal of sufficient amplitude is applied to a Schottky-gate FET, a current can be generated between the gate and the source or drain of the FET. This gate current is essentially independent of the source-to-drain voltage and is therefore not controlled by the basic circuit previously described. When this gate current is excessive, device failures often result, apparently due to voids produced in the gate electrodes.
It is therefore a primary object of the invention to provide a means for preventing excessive gate current during the operation of an FET.